He was, during his lifetime, employed for his engineering and skill of invention. Many of his designs, such as the movable dikes to protect Venice from invasion, proved too costly or impractical. Some of his smaller inventions entered the world of manufacturing unheralded. As an engineer, Leonardo conceived ideas vastly ahead of his own time, conceptually inventing a helicopter, an armoured fighting vehicle, the use of concentrated solar power, a calculator, a rudimentary theory of plate tectonics and the double hull. In practice, he greatly advanced the state of knowledge in the fields of anatomy, astronomy, civil engineering, optics, and the study of water (hydrodynamics).
Leonardo's most famous drawing, the Vitruvian Man, is a study of the proportions of the human body, linking art and science in a single work that has come to represent Renaissance Humanism. Humanism is the study of classical antiquity, at first in Italy, and then spreading across Western Europe in the 14th, 15th and 16th centuries. The terms Renaissance (rinascimento "rebirth") for this movement, and "humanist" (whence modern humanism; also Renaissance humanism to distinguish it from later developments grouped as humanism) are contemporary to that period.
His curiosity and interest in scientific observation were stimulated by his uncle Francesco, while his grandfather's keeping of journals set an example which he was to follow for most of his life, diligently recording in his own journals both the events of the day, his visual observations, his plans and his projects. The journals of Leonardo contain matters as mundane as grocery lists and as remarkable as diagrams for the construction of a flying machine.
As a scientist, Leonardo had no formal education in Latin and mathematics and did not attend a university. Because of these factors, his scientific studies were largely ignored by other scholars. Leonardo's approach to science was one of intense observation and detailed recording, his tools of investigation being almost exclusively his eyes. His journals give insight into his investigative processes.
Leonardo kept a series of journals in which he wrote almost daily, as well as separate notes and sheets of observations, comments and plans. He wrote and drew with his left hand, and most of his writing is in mirror script, which makes it difficult to read. Much has survived to illustrate Leonardo's studies, discoveries and inventions.
On his death, his writings were left mainly to his pupil Melzi with the apparent intention that his scientific work should be published. This did not take place in Melzi's lifetime, and the writings were eventually bound in different forms and dispersed. Some of his works were published as a Treatise on Painting 165 years after his death.